1. Field of the Invention
This invention relates to semi-insulating GaN, and to methods of making same. The semi-insulating GaN material of the invention is useful, inter alia, as a substrate for the manufacture of electronic and/or optoelectronic devices.
2. Description of the Related Art
Gallium nitride and related III–V alloys have exhibited great potential for high temperature and high-frequency electronic applications. However, due to a lack of large area native GaN substrates, most GaN devices have been grown on non-native (heteroepitaxial) substrates such as sapphire and silicon carbide. The use of such foreign substrates is problematic due to lattice mismatch and TE (thermal expansion) mismatch between GaN and the substrate material. One consequence of TE mismatch is bowing of the GaN/heteroepitaxial substrate structure, which leads in turn to cracking and difficulty in fabricating devices with small feature sizes.
Conductive GaN substrates have recently become available (e.g., the conductive GaN substrates that are commercially available from ATMI, Inc., Danbury, Conn. 06810, USA). Such GaN conductive substrates are advantageously employed in applications where the substrate must be conductive and homoepitaxial in relation to associated device structure of GaN. However, in a number of electronic applications such as high frequency electronic applications, a semi-insulating GaN substrate is highly desirable.
U.S. Pat. No. 6,273,948 issued to Porowski et al describes a method of fabricating highly resistive GaN bulk crystals, by crystallization from a solution of atomic nitrogen in a molten mixture of gallium and Group II metal such as beryllium or calcium, under high pressure of 0.5–2.0 GPa and high temperature of 1300–1700° C. Resistivity of 1×104to 1×108 ohm-centimeter (ohm-cm) was achieved. However, the crystal obtained from the process was about 1 cm in size, whereas most commercial electronic applications require a substrate size of at least about 2 inches (>5 cm) diameter.
U.S. Pat. No. 5,686,738 (Moustakas), U.S. Pat. No. 6,544,867 (Webb et al.), U.S. Pat. No. 6,261,931 (Keller et al.), and U.S. patent application Ser. No. 2002/0096692 A1 (Nakamura et al.), disclose various methods of making semi-insulating GaN films on a foreign substrate. All of these approaches are susceptible to TE mismatch issues, and the resultant bowing, cracking and small feature fabrication difficulties discussed above, and none of such approaches has yielded a commercially viable large-area single-crystal semi-insulating gallium nitride material.
There is accordingly a compelling need in the art for large-area semi-insulating GaN substrates.